1. Field of the Invention
The present invention relates to a driving device for an illuminating lamp of a flash apparatus.
2. Description of the Prior Art
There is already known a photographic flash apparatus, particularly flash apparatus for underwater photography, incorporating an illuminating device with a tungsten lamp (hereinafter called modelling light) for illuminating an object to enable proper framing or lens focusing even when the object and the surrounding are dark.
In a flash apparatus, it is well known that the voltage of the power source battery shows a drop due to the internal resistance thereof, since a large current is required at the initial period of charging the main condenser through the voltage booster circuit. Such drop of the battery voltage at the initial period of charging operation is particularly marked in a flash apparatus in which the main condenser is rapidly charged to a determined voltage higher than the dischargeable voltage of the flash tube. Consequently the modelling light composed of a tungsten lamp, if simply connected to the power source of the flash apparatus, will suffer from varying light intensity due to the fluctuation in the battery voltage.
In order to avoid such drawback there has been proposed to employ a large battery with a lower internal resistance, or to serially connect a constant-voltage circuit with the modelling light for providing the same with a constant voltage. However the former solution increases the dimension of the flash apparatus, while the latter is associated with heat generation caused by a voltage drop, since a high power source voltage is reduced to a low voltage in the flash apparatus.